Sign up to receive free email alerts when patent applications with chosen keywords are publishedSIGN UP

Abstract:

An optical receiver assembly that is configured to avoid the introduction
of feedback in an electrical signal converted by the assembly is
disclosed. In one embodiment, an optical receiver assembly is disclosed,
comprising a capacitor, an optical detector provided with a power supply
being mounted on a top electrode of the capacitor, and an amplifier
mounted on the reference surface. The assembly further includes an
isolator interposed between the reference surface and the capacitor,
wherein the isolator includes a bottom layer of dielectric material that
is affixed to a portion of the reference surface, and a metallic top
plate that is electrically coupled both to a ground of the amplifier and
to the capacitor. This configuration bootstraps the amplifier ground to
the amplifier input via the photodiode top electrode of the capacitor to
cancel out feedback signals present at the amplifier ground.

Claims:

1. An optical transceiver module, comprising: a housing; a printed
circuit board at least partially included in the housing; a transmitter
optical subassembly electrically coupled to the printed circuit board;
and a receiver optical subassembly electrically coupled to the printed
circuit board, the receiver optical subassembly including an optical
receiver package, the optical receiver package including: a package base
defining a grounded reference surface; a photodiode configured to receive
and convert an optical signal into an electrical signal, photodiode
electrically coupled to a capacitor; a capacitor having a first electrode
electrically coupled to a first electrode of the photodiode; a
transimpedance amplifier that amplifies the electrical signal produced by
the photodiode, the transimpedance amplifier mounted on the reference
surface and being electrically coupled to the photodiode; and an
isolator, comprising: a dielectric mounted on the reference surface; and
a metallic layer overlying the layer of dielectric material, wherein the
capacitor is affixed to the metallic layer, and wherein the metallic
layer is electrically coupled to a ground of the transimpedance
amplifier.

2. The optical transceiver module as defined in claim 1, wherein the
metallic layer of the isolator is electrically coupled to the ground of
the transimpedance amplifier via at least one wire bond that extends from
the metallic layer to a bond pad of the transimpedance amplifier.

3. The optical transceiver module as defined in claim 2, wherein first
and second wire bonds extend in non-parallel directions between the
metallic layer and bond pads of the transimpedance amplifier.

4. The optical transceiver module as defined in claim 1, wherein a power
supply is supplied to the photodiode via the capacitor.

5. The optical transceiver module as defined in claim 4, wherein the
ground of the transimpedance amplifier is electrically coupled to the
power supply via the metallic layer and the capacitor.

6. An optoelectronic receiver package, comprising: a package base
defining a grounded reference surface; a photodiode mounted on a first
capacitor, the photodiode configured to receive and convert an optical
signal into an electrical signal, a first electrode of the capacitor
being electrically coupled to a first electrode of the photodiode; a
transimpedance amplifier that amplifies the electrical signal produced by
the photodiode, the transimpedance amplifier mounted on the reference
surface, the transimpedance amplifier including an input electrically
coupled to a second electrode of the photodiode; and an isolator,
comprising: a dielectric material that is affixed to a portion of the
reference surface; and a metallic layer atop the dielectric, wherein a
second electrode of the first capacitor is affixed to the metallic layer,
and wherein the metallic layer is electrically coupled to a ground of the
transimpedance amplifier via at least one bond wire.

7. The optoelectronic receiver package as defined in claim 6, wherein the
isolator enables an electrical path to be established between the ground
of the transimpedance amplifier and the input of the transimpedance
amplifier via the metallic layer, the first capacitor, and the
photodiode.

8. The optoelectronic receiver package as defined in claim 15, wherein
the electrical path provides a conduit by which feedback signals present
at the ground of the transimpedance amplifier are transmitted to the
input of the transimpedance amplifier to cancel at least a portion of the
feedback signal.

9. The optoelectronic receiver package as defined in claim 8, wherein the
electrical path is independent of the grounded reference surface.

10. The optoelectronic receiver package as defined in claim 9, wherein
the isolator and the transimpedance amplifier are electrically coupled
via two bond wires that each extend between the isolator metallic layer
and a respective bond pad of the transimpedance amplifier, wherein the
bond wires are to prevent mutual coupling.

11. The optoelectronic receiver package as defined in claim 10, wherein a
power supply for the photodiode is provided by a lead of the package base
and is supplied to a first electrode of the first capacitor, wherein a
decoupling capacitor is interposed between the lead and the first
capacitor.

12. The optoelectronic receiver package as defined in claim 6, wherein
the package is included in an optical transceiver module.

13. The optoelectronic receiver package as defined in claim 12, wherein
the optical transceiver module is configured to receive optical signals
at a rate of at least 10 GHz.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of, and claims priority to, U.S.
patent application Ser. No. 11/676,474, filed Feb. 19, 2007, which claims
the benefit of the U.S. Provisional Patent Application No. 60/774,787,
filed Feb. 17, 2006, and entitled "DISCRETE BOOTSTRAPPING IN AN OPTICAL
RECEIVER TO PREVENT SIGNAL FEEDBACK," each of which is incorporated
herein by reference in its entirety.

BACKGROUND

[0002] 1. Technology Field

[0003] The present invention generally relates to receivers used to detect
optical signals in an optical communications network. In particular, the
present invention relates to a discrete bootstrapping configuration for
an optical receiver that reduces the incidence of feedback of a signal
received by the optical receiver.

[0004] 2. The Related Technology

[0005] Fiber-optics and optoelectronics are important aspects of modern
optical networks because they allow for efficient, accurate and rapid
transmission of optical data between various components in the network
system. An optical transceiver module ("transceiver") is an example of a
modular component that is used in optical networks. Such modular
components are desirable in optical networks and other fiber optic
systems to reduce the cost of manufacturing the system, which cost
increases the more customized the system becomes.

[0006] Transceivers usually include an input receiver optical subassembly
("ROSA") and an output transmitter optical subassembly ("TOSA"). The ROSA
includes a photodiode or other optical detector for detecting optical
signals and sensing circuitry for converting the optical signals to
electrical signals compatible with other network components. The TOSA
includes a laser or other suitable light source for transmitting optical
signals and may include control circuitry for modulating the laser
according to an input digital data signal and a photodetector to monitor
laser power.

[0007] The TOSA has an optical lens for focusing the optical signals from
the laser of the TOSA into an optical fiber. Similarly, the ROSA often
includes a lens to focus incoming optical signals on the photodiode.
Additionally, one end of the transceiver includes pluggable receptacles,
pig-tailed connections, or other suitable means for optically connecting
the TOSA and the ROSA with other components within a fiber optic network,
while an opposite end of the transceiver includes a connector for
connecting with electrical components of a host system or device with
which the transceiver communicates.

[0008] The photodiode in the ROSA and the laser in the TOSA are examples
of optoelectronic semiconductor components. Generally, these
optoelectronic semiconductor components are sensitive devices that
require mechanical and environmental protection. As such, these
optoelectronic components are usually manufactured in packages to provide
such protection and to facilitate their incorporation into higher level
devices, such as TOSAs and ROSAs.

[0009] One such packaging assembly is known as a transistor-outline
package, referred to herein as a "TO package." TO packages are widely
used in the field of optoelectronics, and may be employed in a variety of
applications. As such, TO packages are often standardized to facilitate
their incorporation into components such as transceivers. The TO packages
protect the sensitive electrical devices contained therein and
electrically connect such devices to external components such as printed
circuit boards ("PCB").

[0010] With respect to their construction, the TO packages often include a
cylindrical metallic base, also known as a header, with a number of
conductive leads extending completely through, and generally
perpendicular to, the base. The size of the base is often sized to fit
within a specific TO standard size and lead configuration, examples of
which include a TO-5 or TO-46. The leads are usually hermetically sealed
in the base to provide mechanical and environmental protection for the
components contained in the TO package, and to electrically isolate the
conductive leads from the metallic material of the base. Typically, one
of the conductive leads is a ground lead that may be electrically
connected directly to the base.

[0011] Various types of electrical devices and optical components, such as
the photodiode or laser device, are mounted on an interior portion of the
base and connected to the leads to enable their operation. Generally a
cap, also known as a can, is used to enclose the interior portion of the
base where such electrical devices are mounted so as to form a hermetic
chamber that helps prevent contamination or damage to the devices. The
specific design of the TO package depends on the optoelectronic component
being mounted on the base and the modular component with which the TO
package will be used. By way of example, in applications where the
optoelectronic component mounted on the base is an optical component,
i.e., a laser or photodiode, the cap is at least partially transparent so
as to allow an optical signal generated or received by the optical
component to be transmitted to or from the TO package. These optical TO
can packages are also known as window cans.

[0012] As stated above, optical receivers are specifically built for the
purpose of receiving and interpreting light signals. An optical receiver
typically includes some sort of detector that can generate an electrical
current or voltage in response to changes in the power of the incident
optical signal. After the fiber optic receiver converts the optical
signal received over the optical fiber into an electrical signal, the
optical receiver amplifies the electrical signal, and converts the
electrical signal into an electrical digital data stream.

[0013] One of the common devices used as a detector in an optical receiver
is a photodiode. A photodiode operates by generating a current in
response to incident light. The optical power of the incident light
determines the current that flows in the photodiode. In effect, the
optical signal generates current in the photodiode that corresponds to
the digital data carried by the optical fiber.

[0014] Despite their utility, packages such as TO packages that house
photodiodes or other optical detectors can suffer from
performance-related challenges. One of these challenges is signal
feedback. In the case of optical receiver packages, feedback is a result
of amplification of the electrical signal converted from an optical
signal received by the photodiode, as explained above. This amplification
is performed by a signal amplifier, such as a transimpedance amplifier,
and the amplification of the output signal produced by the amplifier can
be significant when compared to the original strength of the converted
photodiode signal, which can result in a certain amount of feedback.
Moreover, the signals converted by the photodiode are often high
frequency signals of 10 GHz or more, which can further exacerbate
feedback.

[0015] Thus, significant signal amplification, together with the high
frequency of the amplified signal, combine to create a signal that is apt
to produce feedback in the system in which the photodiode and amplifier
are found. This feedback is manifested as a portion of the signal from
the amplifier ground that migrates back to the amplifier input via
various structures, including the header surface, power supply and ground
connections, bond wires, etc. Such feedback is unintended and can
represent a significant limitation in terms of performance of the
package, e.g., frequency response. Should the feedback exceed minimal
levels, oscillation can occur, which undesirably destroys any
functionality of the package and requires scrapping of the part.

[0016] In light of the above, a need exists for controlling feedback in an
optical receiver system, such as an optoelectronic package housing a
photodiode, in order to optimize operation of the device. Any solution
should be implemented in a manner that does not substantially increase
the sophistication or complexity of the device and that does not
compromise signal integrity.

BRIEF SUMMARY

[0017] The present invention has been developed in response to the above
and other needs in the art. Briefly summarized, embodiments of the
present invention are directed to an optical receiver assembly that is
configured to avoid the introduction of feedback in an electrical signal
converted by the assembly. In one embodiment, an optical receiver
assembly is disclosed, comprising a capacitor, an optical detector
provided with a power supply being mounted on the capacitor, and an
amplifier mounted on a top electrode of the reference surface. The
assembly further includes an isolator interposed between the reference
surface and the capacitor. The isolator includes a bottom layer of
dielectric material that is affixed to a portion of the reference surface
and a metallic top plate that is electrically coupled both to a ground of
the amplifier and to the capacitor. This configuration "bootstraps" the
amplifier ground to the amplifier input via the photodiode and top
electrode of the capacitor, and by so doing, allows feedback signals
present at the amplifier ground to be transmitted to the amplifier input
via the photodiode, which desirably cancels the feedback signal in the
circuit.

[0018] Configuration of the optical receiver assembly as described above
in one embodiment further reduces parasitic capacitance and inductance
that may otherwise be present in the circuits of the assembly.

[0019] The optical receiver assembly in one embodiment forms part of an
optoelectronic package housed in an optical subassembly for use within an
optical transceiver module, for instance. As such, the optical receiver
can form an integral part of an optical communications network.

[0020] These and other features of the present invention will become more
fully apparent from the following description and appended claims, or may
be learned by the practice of the invention as set forth hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] To further clarify the above and other advantages and features of
the present invention, a more particular description of the invention
will be rendered by reference to specific embodiments thereof that are
illustrated in the appended drawings. It is appreciated that these
drawings depict only typical embodiments of the invention and are
therefore not to be considered limiting of its scope. The invention will
be described and explained with additional specificity and detail through
the use of the accompanying drawings in which:

[0022] FIG. 1 is a perspective view of an optical transceiver module which
serves as one exemplary environment in which embodiments of the present
invention can be practiced;

[0023]FIG. 2 is a simplified block diagram of an optical receiver that
can include a photodiode to TIA interface configured in accordance with
one embodiment of the present invention;

[0024] FIG. 3 is a perspective view of a base portion of an optoelectronic
package, including an optical receiver assembly in accordance with one
embodiment;

[0025] FIG. 4 is a schematic diagram showing various electrical aspects of
the optical receiver assembly shown in FIG. 3; and

[0026]FIG. 5 is a graph plotting various parameters associated with
operation of the optical receiver assembly configured in accordance with
one embodiment of the present invention.

DETAILED DESCRIPTION OF SELECTED EMBODIMENTS

[0027] Reference will now be made to figures wherein like structures will
be provided with like reference designations. It is understood that the
drawings are diagrammatic and schematic representations of exemplary
embodiments of the invention, and are not limiting of the present
invention nor are they necessarily drawn to scale.

[0028] FIGS. 1-5 depict various features of embodiments of the present
invention, which is generally directed to an optical receiver assembly
that is configured to avoid the introduction of feedback in an electrical
signal converted by the assembly. Reduction or elimination of signal
feedback in the optical receiver assembly enables optimization of the
assembly and assurance of acceptable signal integrity for its use within
a communications network, for example. Embodiments of the optical
receiver assembly can be included in optoelectronic packages, including
TO packages that form a component of an optical subassembly for an
optical transceiver module. Such modules are central to the interfacing
of electronic devices, such as computers, routers and the like, with an
optical communications network employing fiber optic technology. In
addition, the feedback-reducing optical receiver assembly can also be
employed in other operating environments if desired.

[0029] As mentioned, an exemplary embodiment of the optical receiver
assembly described herein is embodied within an optoelectronic package of
a receiver optical subassembly ("ROSA") for an optical transceiver module
("transceiver"). The ROSA, together with a transmitter optical
subassembly ("TOSA") of the transceiver, includes various components to
enable the reception and transmission of optical signals to and from a
host system that is operably connected to the transceiver. The host
system can be included as a node in an optical communications network,
for instance, and can employ the transceiver in communicating via optical
signals with other components of the network. Note, however, that the
discussion to follow regarding embodiments of the present invention as
they relate to controlling feedback in relation to an optical receiver
should not be construed as limiting the present invention to only such
embodiments. Indeed, it is appreciated that principles of the present
invention can extend to optical receivers employed in other
configurations as well.

[0030] Reference is first made to FIG. 1, which depicts a perspective view
of an optical transceiver module ("transceiver"), generally designated at
100, for use in transmitting and receiving optical signals in connection
with an external host that is operatively connected in one embodiment to
a communications network (not shown). As depicted, the transceiver shown
in FIG. 1 includes various components, including an optical receiver
implemented as a receiver optical subassembly ("ROSA") 20, a transmitter
optical subassembly ("TOSA") 10, electrical interfaces 30, various
electronic components 40, and a printed circuit board 50. In detail, two
electrical interfaces 30 are included in the transceiver 100, one each
used to electrically connect the ROSA 20 and the TOSA 10 to a plurality
of conductive pads located on the PCB 50. The electronic components 40
are also operably attached to the PCB 50. An edge connector 60 is located
on an end of the PCB 50 to enable the transceiver 100 to electrically
interface with a host (not shown here). As such, the PCB 50 facilitates
electrical communication between the TOSA 10/ROSA 20, and the host. In
addition, the above-mentioned components of the transceiver 100 are
partially housed within a housing portion 70. Though not shown, a shell
can cooperate with the housing portion 70 to define a covering for the
components of the transceiver 100.

[0031]FIG. 2 illustrates further details regarding the exemplary
environment of FIG. 1 for implementing embodiments of the present
invention. In detail, FIG. 1 illustrates in block form a fiber optic
receiver 101 that includes components that are found in the ROSA 20 and
electronic components 40 of the PCB 50 shown in FIG. 1. The receiver 101
receives a data-containing optical signal ("light") 103 over an optical
fiber 102. A photodiode 104 or other optical device for converting an
optical signal receives the optical signal and converts it into an
electrical signal 106, manifested as an electrical current. A
transimpedance amplifier ("TIA") 108 amplifies the electrical signal 106
to produce the amplified electrical signal 110. The TIA 108 has a wide
dynamic range that is able to amplify signals with large power without
significantly diminishing the ability to amplify signals with low power.
The amplified electrical signal 110 is then amplified by a post amplifier
112 or is operated on by another integrated circuit such as a clock and
data recovery circuit. The output 114 of the post amplifier 114 is
interpreted or translated by the translation module 116 and converted
into an electrical digital signal 118. The digital signal 118 can then be
forward to a host via other components of the transceiver for use by the
host.

[0032] Reference is now made to FIG. 3 in describing details regarding an
exemplary embodiment of the present invention. In particular, FIG. 3
shows portions of an optical receiver package, generally designated at
150. The package 150 is a TO package and includes a base 152 that is
configured to mate with a cap (not shown) in order to form a hermetic
environment in which other components of the package 150 can reside.
Various leads 154A-D extend through glass seals 156 of the base in order
to provide electrical communication between package components and
devices positioned exterior to the package 150. The glass seals both
environmentally and electrically isolate the leads 154A-D as to prevent
electrical shorting or contamination of the hermetic package environment.
A reference surface 158 is included on the base and serves as a platform
for mounting various package components. When the cap is installed on the
base 152 the reference surface 158 and any components mounted thereon are
included within the hermetic environment of the package 150. The base 152
is composed of a metallic substance and is grounded via a lead (not
shown) such that the reference surface serves as a ground plane for
various of the components of the package 150.

[0033] The package 150 further includes an optical receiver assembly,
generally designated at 200, which is configured in accordance with one
embodiment of the present invention. The assembly 200 generally includes
an optical detector, such as a photodiode ("PD") 202, and a signal
amplifier, such as a transimpedance amplifier ("TIA") 204. The PD 202 in
one embodiment is an avalanche photodiode and is positioned in the
package 150 to receive incident light from an optical fiber in the manner
generally depicted in FIG. 2 and to convert the light into an electrical
signal. The TIA 204 is operably connected to the PD 202 in order to
receive the electrical signal produced by the PD and amplify the signal
prior to forwarding it to other transceiver components, as shown in FIG.
2.

[0034] In particular, the TIA 204 includes various bond pads 206 to allow
interconnection of the TIA with various other package components. One of
the bond pads 206A is employed to electrically connect with a
corresponding bond pad on a top surface 202A of the PD 202 via a bond
wire 208. In addition to this configuration, other alternative
configurations can be employed to electrically connect the PD 202 and TIA
204 together.

[0035] A capacitor 210 is included in the package 150 to reduce the
injection of noise into the electrical signal produced by the PD 202. As
shown in FIG. 3, the PD 202 is affixed to a surface of the capacitor 210.
As such, a bottom electrode 202B of the PD 202 is electrically coupled to
top electrode 210A that defines the top surface of the capacitor 210,
thereby electrically coupling the PD and capacitor together. A bottom
surface of the capacitor 210 defines a bottom electrode 210B of the
capacitor 210, which electrode is electrically coupled to a top plate 254
of the isolator 250. In the present embodiment, the capacitor 210 is a
single layer capacitor, though in other embodiments other suitable
capacitor types could be employed.

[0036] Electrical power supplies are provided to both the PD 202 and the
TIA 204 in order to enable their functionality. Specifically, the lead
154A provides a power supply both to the PD 202 and the electrode 210A of
capacitor 212A, which capacitor is positioned on the reference surface
158 of the base 152, via bond wires 214A. The second electrode of the
capacitor being upon its lower surface which is in contact with reference
surface 158 and which is electrically coupled to reference surface 158.
Specifically, one of the bond wires 214A is attached to and extends
between a capacitor 212A and the top electrode 210A of the capacitor 210
on which the PD 202 is positioned. In this way, a power supply signal is
provided both to the bottom electrode 202B of the PD 202 and the top
electrode 210A of the capacitor 210 via the bond wires 214A. In addition,
the capacitor 212A is also powered by the bond wire 214A extending from
the lead 154A, together with connection of a bottom electrode of the
capacitor 212A with the reference surface 158.

[0037] Similarly, the lead 154B provides a power supply to the TIA 204 via
bond wires 214B and a top portion of an interposed capacitor 212B
positioned on the reference surface 158. Specifically, one of the bond
wires 214B is attached to and extends between a top electrode of the
capacitor 212B and one of the bond pads 206 of the TIA 204, while two
additional bond wires 214B extend between the capacitor top surface and
the lead 154B. A bottom electrode of the capacitor 212B included on a
bottom surface of the capacitor is electrically coupled to the reference
surface 158. In this way, a power supply signal is provided the TIA 204
via the bond wires 214B and the top electrode of the capacitor 212B. Note
that the capacitor 212A is used in the present embodiment as described
above such that any time-varying voltage present on its top electrode is
pasted through the electrode to the reference surface 158.

[0038] As already mentioned, the PD 202 provides an electrical signal
representative of an optical signal received thereby, and forwards the
electrical signal to the TIA 204 for amplification. Once amplified, the
electrical signal is forwarded to the package leads 154C and 154D via
respective bond wires 216 as a differential signal. The leads 154C and
154D are configured to convey the differential electrical signals to
other components of the transceiver, such as the post amplifier 112,
shown in FIG. 2, for further processing before being forwarded to a host
or other suitable destination. Note that the TIA 204 further includes a
plurality of bond wires 218 that each extend from various of the bond
pads 206 to the reference surface 158 in order to provide a ground path
for portions of the TIA.

[0039] In accordance with one embodiment, the optical receiver assembly
200 of the package 150 further includes an isolator 250 on which the
capacitor 210 is affixed. The isolator 250 is in turn affixed to the
reference surface 158 of the package base 152. In detail, the isolator
250 includes a bottom layer 252 composed of a low dielectric
material--having a dielectric constant of less than 10 in one
embodiment--and having suitable outgassing properties, i.e., the material
will not release gases during operation that may interfere with operation
of the optical receiver. The isolator 250 further includes a top plate
254 mated with the bottom layer and composed of a conductive material,
such as any suitable metal. Indeed, in one embodiment, the bottom layer
252 is composed of aluminum oxide, i.e., alumina, while the top plate 254
is gold. Alternative materials from which the isolator bottom layer can
be composed include fused silica and aluminum nitride. Variations in the
composition of the isolator components are possible, however, in
accordance with the requirements set forth herein.

[0040] In greater detail, the bottom layer 252 of the isolator 250 is
affixed to the reference surface 158 of the package base 152. In turn,
the bottom electrode 210B of the capacitor 210 is affixed to the top
plate 254 of the isolator, and the bottom electrode 202B of the PD 202 is
affixed to the capacitor. In addition, bond wires 256 extend from ground
bond pads 206B of the TIA 204 to the top plate 254, thereby electrically
coupling the TIA ground to the isolator top plate, which in turn couples
the TIA ground to the PD power supply that couples to the capacitor 210.
As will be seen, this isolator configuration effectively cancels out
feedback that may be present in the optical receiver assembly 200.

[0041] Note that the bond wires 256 extend between the TIA 204 and the
isolator 250 in non-parallel directions. This helps prevent mutual
coupling between the bond wires 256, which in turn reduces undesired
circuit inductance.

[0042] Reference is now made to FIG. 4, which includes a circuit diagram
300 depicting various components and characteristics of the optical
receiver assembly and package configured as depicted in FIG. 3. As shown,
the PD 202, TIA 204, and capacitor 210 are depicted in electrical
relation to one another. The TIA 204 includes a signal input indicated at
204A, corresponding to the TIA bond pad 206 in FIG. 3, and a ground
indicated at 204B, which corresponds to the TIA bond pads 206B. A power
supply, Vpd, is shown connected to the PD 202. Residual
capacitances, which represent artifacts of the configuration of the
optical receiver assembly, are depicted at 302 in relation to the
differential electrical signals that are emitted by the TIA 204 and
transmitted via the bond wires 216 and leads 154C and 154D (see FIG. 4).
A parasitic capacitance 304 is also shown. The capacitances 302 and 304
represent leakage capacitances that are present in the system, i.e.,
unintended capacitances occurring between the reference surface 158 of
the package base 152 and components of the optical receiver assembly 200
shown in FIG. 3.

[0043] Various unintended inductances 306 are shown in FIG. 4, which each
represent inherent inductances existing in the various bond wires present
in the optical receiver assembly 200 of FIG. 3. Also shown is a parasitic
inductance 308 that exists between the PD 202 and the TIA 204. In order
to minimize feedback and optimize performance of the optical receiver
assembly 200 in light of embodiments of the present invention, it is
desired to minimize the prevalence of the parasitic capacitance 304 and
inductance 308.

[0044] With continuing reference to FIGS. 3 and 4, embodiments of the
present invention feature the ability to control the presence of feedback
in the electrical signal produced and processed by the optical receiver
assembly 200 depicted in FIG. 3. During operation of the optical receiver
assembly 200, extraneous signals related to the amplified electrical
signal produced and output via the bond pads 206C of the TIA 204 for
transmission via the leads 154C and 154D can be undesirably communicated
through various paths in the package 150 or its components to the TIA
input, defined here as the voltage present at bond pad 206A of the TIA
relative to the TIA ground. The paths and components by which these
extraneous signals can travel include grounds paths internal to the TIA
204, the reference surface 158 of the package base 152, the power supply
capacitors 212A and 212B, etc.

[0045] This extraneous signal pollution is caused in part by system
geometry and the high frequency and amplified strength of the TIA signal.
The introduction of this extraneous signal in the above manner results in
a net signal being present at the TIA input at bond pad 206A relative to
the TIA internal ground, i.e., a feedback signal. As has been discussed,
such feedback can inhibit operation of the optical receiver assembly and
hinder the desired electrical signal it produces.

[0046] In greater detail, the TIA ground itself provides an efficient path
for transferring a feedback signal from the TIA outputs at TIA bond pads
206C back to the TIA input at bond pad 206A due to the high conductivity,
short length, and low path inductance of the TIA ground. Any imbalance in
the loads present at the two differential TIA output bond pads 206C or in
the TIA output signals themselves will result in a net signal being
induced in the TIA ground. Such a TIA ground signal can be coupled back
to the TIA input at bond pad 206A with high efficiency. Put another way,
because the TIA input signal is interpreted relative to the TIA ground,
any signal present on the TIA ground relative to the TIA input at the
bond pad 206A will carry the same importance as if that signal were
present directly on the TIA input. Thus, any feedback signal present on
the TIA ground, if not compensated for, will interfere with intended
signals received at the TIA input at the bond pad 206A. In order to
cancel the effect of the TIA ground signal on the TIA input, it is
desirable to couple the TIA ground signal back to the TIA input bond pad
206A itself in an efficient manner, in accordance with embodiments of the
present invention.

[0047] The optical receiver assembly configuration shown in FIGS. 3 and 4
is configured to control such signal feedback via the use of a
"bootstrapping" electrical path configuration, which results in the
ground of the TIA 204 being electrically coupled to the ground of the PD
202, and ultimately to the TIA input 206A. This bootstrapping path is
defined in FIG. 3 by the bond pads 206B of the TIA, which are coupled to
the internal TIA ground, being electrically coupled with the top plate
254 of the isolator 250 via the bond wires 256. Note that the isolator
bottom layer 252 prevents this TIA ground from electrically connecting
with the reference surface 158. The isolator top plate 254, in turn, is
electrically coupled to the capacitor 210, which is coupled to the PD
202, as has been described. Also as described, the power supply for
supplying power to the PD 202 is delivered via the bond wires 214A, one
of which connects with the top electrode 210A of the capacitor 210.

[0048] The above configuration establishes the electrical bootstrap path
from the TIA ground to the PD ground that is independent of the package
base reference surface 158. In another sense and as shown in FIG. 4, the
TIA ground 204B is tied to the power supply Vpd via the capacitor
210, again enabling a tie-in between the TIA ground 204B and the TIA
input 204A via the PD 202. Note that, despite the above description the
bootstrapping configuration can be realized via other components and
connections that preserve the functionality of the configuration
described herein.

[0049] The above bootstrapping configuration accounts for the
above-described feedback and prevents it from compromising the integrity
of the amplified electrical signal that is produced, amplified, and
forwarded by the optical receiver assembly 200. In particular, any
extraneous feedback signal that is undesirably acquired by the TIA ground
as described above can be effectively transmitted, via the above
bootstrapping configuration, to the bottom surface of the capacitor 210
that is affixed to the top plate 254 of the isolator 250. Because the
capacitor 210 lacks significant impedance at the frequencies of interest,
it allows the extraneous signal to pass through the capacitor and reach
the PD 202. The extraneous signal can then be forwarded by the PD 202 to
the TIA input bond pad 206A, together with the desired electrical signals
typically produced by the PD during reception of optical signals. Thus,
any extraneous feedback signal present at the TIA ground is also
forwarded by the bootstrapping configuration such that it is also present
at the TIA input. The presence of the extraneous feedback signal at both
the TIA input and ground effectively cancels the signal out of the TIA
input stream. This leaves only the desired signal to be amplified and
forwarded by the TIA 204.

[0050] The above canceling effect is made possible by the bootstrapping
configuration described above together with placement of the isolator
250, as the isolator prevents the PD 202 from electrically coupling with
the reference surface 158 as a ground source. Rather, the PD 202
electrically couples with the TIA ground via the bootstrapping
configuration, as already described. Also, the bootstrapping
configuration, together with the isolator 250, advantageously reduces or
eliminates the parasitic inductance 308 and capacitance 304 of the
assembly as depicted in FIG. 4 by virtue of their mutual design. This in
turn enables signals to travel unimpeded between the TIA ground at 206B
to the TIA input at 206A via the capacitor 210 and PD 202. This desirably
permits the extraneous feedback signal canceling effect described above
to occur.

[0051]FIG. 5 is a graph that depicts modeled results that demonstrate the
beneficial results of the above feedback canceling effect, wherein graphs
502A and 502B show the amount of signal output from the amplifier 204
that migrates back to the positive and negative input of the amplifier,
respectively, and graph 504 is the combination of these two migrated
signals as received and amplified by the amplifier. Graph 504 shows that
the amount of amplified feedback, or gain, is desirably below the amount
where feedback normally occurs.

[0052] The particular characteristics of the isolator 250 can be
configured to provide the desired performance thereof. Thus, for example,
the top surface areal size, dielectric constant, and thickness of the
bottom layer 252 of the isolator 250 can be altered as needed to provide
sufficient isolation for the top plate 254, given the fact that
performance of the isolator is inversely proportional to its capacitance,
which in turn is proportional both to the areal size of the top surface
and its dielectric constant and inversely proportional to its thickness.

[0053] In accordance with the foregoing, the bootstrapping configuration
discussed above discloses one exemplary means for electrically coupling a
ground of an amplifier, such as the TIA 204, to an input of the
amplifier, independent of a reference surface in view of controlling
feedback in an optical receiver assembly. However, as noted these
structures are simply one example of a means for such electrical
coupling. Indeed, other structures and components could be implemented to
accomplish the same functionality as that described herein. Thus, the
above disclosure should not be considered limiting of the present
invention in any way.

[0054] The present invention may be embodied in other specific forms
without departing from its spirit or essential characteristics. The
described embodiments are to be considered in all respects only as
illustrative, not restrictive. The scope of the invention is, therefore,
indicated by the appended claims rather than by the foregoing
description. All changes that come within the meaning and range of
equivalency of the claims are to be embraced within their scope.